松散土体中细颗粒运移的微观过程研究

Study on Microscopic Process of Fine Particle Migration in Loose Soil

自然界的松散土是泥石流、滑坡等灾害发生的重要物质之一,其结构的松散性和长期降雨渗流作用为细颗粒发生运移、形成内部侵蚀提供了空间和动力条件,颗粒大量流失和孔隙通道堵塞造成土体结构变化和稳定性下降,从而演变为滑坡或坍塌。渗透、水槽和人工降雨等试验方法对认识土体内部细颗粒运移的宏观特征具有重要作用,但无法直接分析孔隙通道内细颗粒分布、位移等随时间的变化特征。作者结合上海同步辐射光源3维CT技术,为获得拟静态下1维柱体渗流过程中细颗粒侵蚀形态特征,以不同粗细颗粒粒径之比为变量设计微观渗流试验,通过耦合离散元与Darcy流体方程计算分析整体和局部区域内细颗粒数量和平均动能的变化特征。结果表明:离散元与Darcy流耦合是计算土体内部细颗粒运移的有效手段。CT扫描和数值计算结果均表明土样入流口和出流口存在优先侵蚀现象。计算至2.5 s时,已分别有37.05%和31.95%细颗粒被侵蚀,其他位置侵蚀程度相对较低。在渗流方向上土体内部细颗粒存在流失补给平衡和逐渐侵蚀的现象,细颗粒的平均动能沿渗流方向总体呈逐渐增高的趋势。长期性堵塞形成过程中,细颗粒的平均动能呈现随时间逐渐降低的趋势;临时性堵塞区域内细颗粒数量的增加相对于此区域内细颗粒平均动能的增高存在滞后效应。微观尺度上土体内部细颗粒运移特征主要受流体状态和孔隙特征影响,其研究对于理解松散土坡体破坏机制具有重要价值。

Field observations show that loose soils are common deposit in gullies after an earthquake which provides ideal sources of material for debris flows and landslides.During rainfall infiltration process,pore structure provided by coarse soil skeleton provids a natural flow channel inside soil for fine particles to migrate,thus degrading the stability of soil structure and causing further slope failure.Pervious study of fine particle migration in soil mainly focused on seepage experiments,mid-scale flume test with rainfall as a boundary condition,and other macro-scale methods.However,these methods could not directly obtain the parameters such as pore structure,velocity of fine particles,and pore pressure inside soil sample to quantify the internal erosion process.In the current study,a series of one-dimensional cylinder microscopic seepage tests were designed,the coarse to fine particle size ratio was chosen as a controlling parameter,the three-dimensional computed tomography technology in Shanghai Synchrotron Radiation Facility (SSRF) was then used to quantify the characteristics of fine particle migration during seepage process.The numerical simulation using discrete element method (DEM) coupled with Darcy’s flow was then used to back analysis the physical test.The results of CT and numerical calculations revealed that there existed preferential erosion in the inflow and outflow region of the soil sample.When the numerical simulation is calculated to 2.5s,37.05% and 31.95% of the fine particles had been eroded in above two regions,respectively,while the erosion at other locations is relatively low.The form of fine particle erosion inside the soil in the direction of seepage includs the loss balance of loss and supply and gradual erosion.The average kinetic energy of the fine particles tends to increase along the seepage path.On the one hand,the forming of long-term clogging to flow channels is reflected by the decreasing of average kinetic energy and quantity of fine particles with time;On the other hand,the temporary clogging to flow channels is reflected by the rate of increase of the number of fine particles in the measurement region larger than the rate of increase of average kinetic energy.The fine particle migration characteristics in the soil on the microscopic scale are mainly affected by the fluid state and pore characteristics,and its research is of great value for understanding the mechanism of loose soil slope failure.